With a conventional WCDMA downlink transmission scheme, a spreading code is generated by combining    (1) a scrambling code using a long-period pseudorandom sequence; and    (2) a channelization code using an orthogonal code.
A channelization code is used to identify each of the physical channels used in a sector, and a set of mutually orthogonal codes (the number of which generally equals a spreading factor) is allocated to each of the sectors.
A scrambling code is a unique one allocated to each of the sectors, and it has a long period as long as 10 ms frame length.
FIG. 1 is a schematic diagram illustrating an example of spreading codes allocated to each of the sectors in a WCDMA radio access scheme. The cell 1 managed by base station (BS) 1 is divided into sectors 1-1, 1-2, and 1-3. Each of the sectors 1-1, 1-2, and 1-3 is assigned one of unique scrambling codes S1, S2 and S3. By allocating different scrambling codes to different sectors, the number of accessing users (i.e., the system capacity) can be increased.
Because different scrambling codes are allocated to each of the sectors 1-1 through 1-3 in a cell, a set of channelization codes a1-a4 is commonly used in the sectors 1-1 through 1-3. The number of channelization codes, four in this example, is on the assumption of the spreading factor 4.
Similarly, in cell 2 managed by the base station (BS) 2, a set of channelization codes a1-a4 is commonly used in sectors 2-1 through 2-3, while unique scrambling codes S4, S5, and S6 are allocated to the sectors 2-1, 2-2, and 2-3, respectively. The same applies to cell 3 managed by the base station (BS) 3, in which a set of channelization codes a1-a4 is commonly used in the sectors 3-1 through 3-3, while unique scrambling codes S7, S8, and S9 are allocated to the sectors 3-1, 3-2, and 3-3, respectively.
In this manner, the influence on a downlink signal transmitted in a sector due to interference from other sectors under the same base station and interference from adjacent cells is randomized by multiplying the transmission signal by a scrambling code unique to the sector.
However, under a sectorized cell architecture, the orthogonality of the orthogonal codes may be degraded due to the multiplication of the scrambling code unique to a sector, and as a result, intersector interference may arise. To be more precise, assuming that, in cell 1, channelization code a1 is allocated to user 1 located in sector 1-1 and channelization code a2 is allocated to user 2 located in sector 1-2, then, spreading code C1 of the user 1 and the spreading code C2 of the user 2 are expressed asC1=a1*S1, andC2=a2*S2.
In this case, if receiving timings of signals transmitted from different sectors of the same base station are the same, or if the difference in the receiving timings is absorbed in the guard interval under OFDM radio access, the orthogonality achieved by the channelization codes a1 and a2 is impaired due to the multiplication by the individual scrambling codes, and non-orthogonal code sequences are produced. Consequently, intersector interference occurs under the same base station, and the intended signal transmission property may be degraded.